Blender pure video editor - overlay image with transparent areas
Found it out.

When you add an image to the VSE (video sequence editor, where videos, audios, images, etc. appear as bars like a timeline), click the image bar with the right button. A properties panel will appear for that image, containing the filename and other information about it. 

Under the "Edit Strip" section of that panel, change the "Blend" combobox to "Over Drop". (Usually it starts with "cross"). 

That will make the image transparent areas actually transparent on the video.